Method for producing a strengthened glass structural member

ABSTRACT

The present invention relates to a new glass material applicable in those situations where thermal and/or mechanical shock would limit the use of other glass materials presently available. Dopants are deposited into the interconnected pores of a porous glass in a non-uniform manner such that upon consolidation and cooling the final article has its surface under compressive stress. Dopants may also be added to control color and other appearance features. A porous silicate glass is washed with sodium hydroxide followed by immersing the washed glass in a liquid solution of a dopant in a liquid solvent therefor to stuff the pores of the washed glass with the solution. Thereafter, the solvent is removed from the pores and the pores are collapsed by a heating step.

The invention herein described was made in the course of or under acontract or subcontract thereunder (or grant) with the Department of theAir Force.

This application is a division of our copending application Ser. No.917,101, filed June 19, 1978, now U.S. Pat. No. 4,220,682, which in turnis a division of our U.S. patent application Ser. No. 635,727, filedNov. 26, 1975, now U.S. Pat. No. 4,110,096, which in turn is aContinuation-in-Part of our U.S. patent applications Ser. No. 462,481,filed Apr. 22, 1974, now U.S. Pat. No. 3,938,974 and Ser. No. 559,512,filed Mar. 18, 1975, now abandoned, and said U.S. Ser. No. 462,481 is aContinuation-in-Part of our earlier filed U.S. Ser. No. 355,164, filedApr. 27, 1973, now abandoned.

The present invention relates to a new glass material applicable inthose situations where thermal shock would cause ordinary glasses orceramics to fracture or where increased strength is required independentof temperature. (Note: In this disclosure glass is meant to covercompletely vitreous or partially devitrified glasses, e.g., ceramics.)

Ceramics have long been considered a useful material because of theirease of fabrication and relative low cost. Two of their most significantlimitations are their inability to withstand thermal shock and theirrelatively low strength. This limits the use of such ceramics for manystructural purposes and as cooking utensils, stove tops, radar domes,windshields, windows, and containers such as pipes and tubes, andstorage or reaction vats for use in high temperature processing ofchemicals and other materials.

The standard method for overcoming the thermal shock weakness has beento use or develop ceramics with low thermal expansivity. At presentthere are three feasible approaches: (1) the use of pure fused quartz,(2) the use of Vycor*, and finally (3) the use of glass-ceramics. Thelast approach is also useful for increasing strength.

Each of the above has its limitations. The first two have such lowmodulus of rupture that in many applications they cannot be used. Thethird process (glass-ceramics) has all the desired thermo-mechanicalproperties but is dependent upon a rather complicated production processwhich makes it relatively expensive to produce.

It is a primary object of this invention to provide a new glass materialwith desirable thermo-mechanical properties and a method foreconomically producing same.

The desirable properties are:

(1) high modulus of rupture relative to Vycor, fused silica, Pyrex* andmost other common glass materials;

(2) very low thermal expansivity, smaller than Pyrex;

(3) high upper use temperature superior to ceramics and glasses such asPyrex;

(4)controllable appearance such that it can be made in forms which aretransparent, opalescent, or any color;

(5) can be adapted to relatively low cost production techniques; and

(6) can be used as cooking utensils, stove tops, radar domes, storagevats, pipes, and other places where high strength-to-weight ratio,chemical inertness, and high use temperatures are needed.

An inexpensive method of making glass with high silica content (and thusa low thermal expansivity) was first developed by Hood et al., U.S. Pat.Nos. 2,106,744 and 2,215,036. In this process a sodium borosilicateglass, which may be cast into any desired shape, is heat treated so thatit undergoes phase separation with an interconnected micro-structure.The alkali borate phase is ionic in nature and soluble in acids such asHCl, while the other phase consists mostly of covalently bonded silicacontaining a small amount of B₂ O₃ and is insoluble in HCl. Hence thealkali borate phase may be removed by leaching with acid, leaving behindthe silica rich skeleton. In the Hood process this silica skeleton iscollapsed by heating it to a temperature near T_(g) (about 950° C.),giving a homogeneous glass of high silica content, good chemicaldurability, high use temperature and low expansion coefficient.

In a previous application (U.S. Ser. No. 462,481, filed Apr. 22, 1974,now U.S. Pat. No. 3,938,974.) we have described a process in which aphase-separable glass is converted to a porous form. This porous form isthen converted to a solid glass article with either a uniform ornon-uniform composition profile across at least one cross-sectionalaxis, by adding modifying components to the porous material, andcollapsing the article thus formed into a solid glass article. We havecalled the process of adding such composition modifying components"molecular stuffing."

We have found that such a process is not only applicable to the stuffingof porous matrices produced by the leaching of phase-separated glasses,but is also applicable to other inter-connective porous structureshaving a matrix constituted of at least one glass network formingmaterial. One well-known process for forming interconnective porousstructures other than the phase separation route is by chemical vapordeposition. A convenient description of such a process is contained inU.S. Pat. No. 2,272,342, issued to J. F. Hyde and U.S. Pat. No.2,326,059 issued to R. E. Nordberg. More particularly, U.S. Pat. No.3,859,073 issued to P. C. Schultz describes the formation of a porousbody and its subsequent impregnation with a dopant. Such impregnation isconcerned with the deposition of small quantities of materials fromrelatively dilute solutions in the pores of the porous body.

In the present invention, molecular stuffing is used to improve thestrength of glass material by introducing a composition profile acrossthe thickness of the glass which gives rise in turn to a profile inthermo-mechanical properties. From now on such a composition profilewhich is designed to increase the thermal shock resistance and themechanical strength of a material will be referred to as "thestrengthening profile.

FIGURE CAPTIONS

In the drawings, FIG. 1 is a cross-section of a strengthened glass,showing regions (A) which have a different concentration than region(B).

FIG. 2 is a plot of volume versus temperature showing different glasstransition temperatures for regions A and B.

FIG. 3 is a plot of volume versus temperature showing different thermalexpansion coefficients below the glass transition temperature forregions A and B.

FIG. 4 shows schematically (a) stepped profile and (b) a continuousprofile.

FIG. 5 is a plot of normalized weight gain versus time for stuffing of aporous rod with concentrated solution of CsNO₃ in water at 100° C.

FIG. 6 shows a plot of normalized weight loss versus time for unstuffing(with water at 100° C.) a rod originally stuffed with CsNO₃ solution at100° C.

FIG. 7 shows composition profiles measured in terms of refractive indexdistribution, for two rods unstuffed at 95° C. for periods of 11(Curve 1) and 20 (Curve 2) minutes respectively.

FIG. 8 shows composition profiles measured in terms of refractive indexdistributions for stuffed rods heated under vacuum at rates of (Curve 1)15, (Curve 2) 30 and (Curve 3) 50° C./hour respectively.

There are three ways of using the present invention to create astrengthening profile in a glass material

1. In the first method, which from now on we shall call the T_(g)method, using the present invention one can create a profile in theglass transition temperature, T_(g). Such a composition profile is shownin FIG. 1 schematically, where region B is the silica phase doped bymolecular stuffing and region A is undoped, relatively pure silica. Mostnetwork modifiers when dissolved in pure silica act to lower the glasstransition temperature, T_(g). As determined by thermal expansion, thedifference in T_(g) between the relatively pure silica region A and thedoped silica region B has roughly the effect on the volume temperaturecurves of the two compositions shown in FIG. 2. (Note the small amountof B₂ O₃ actually present in glass A is sufficient to repress thenegative expansion region that would be observed above T_(gA) forextremely pure silica. Hence we refer here to glass A as "pure" silicaonly in comparison to doped glass B.)

As shown in FIG. 2, when the material in FIG. 1 is cooled from theliquid, the pure silica composition A on the surface passes through theglass transition at T_(gA), becoming rigid and experiences a largedecrease in thermal expansion coefficient. The doped silica compositionB in the interior, however, continues to contract with a liquid-likeexpansion coefficient down to the temperature T_(gB) at which point italso becomes a rigid glass with low thermal expansion coefficient. Thedifferential expansion between surface and interior in the temperaturerange T_(gA) to T_(gB) has the effect of placing the surface under acompressive stress. This compressive stress must be overcome before thesurface can undergo fracture. Thus the T_(g) profile achieved bymolecular stuffing of silica results in the strengthening of the surfaceof the glass.

2. In the second method, which we shall, from now on, call "theexpansion-coefficient" method, using the present invention one cancreate a profile in the expansion coefficient.

The dopant in region B (FIG. 1) does not change the glass transitiontemperature T_(g), but increases the expansion coefficient of the glass.Thus upon cooling the sample to room temperature from its glasstransition temperature, region B shrinks by a greater amount than A,resulting in a compressive stress in A, which stress will increase themodulus of rupture of the sample. (See FIG. 3.)

3. In the third method, which we shall, from now on, call "mixedmethod," using the present invention one can create simultaneously aprofile in T_(g) and a profile in the expansion coefficient. Mixedmethod is, therefore, a combination of the previous two methods.

The molecular stuffing strengthening process in this invention hascertain similarities to chemical tempering by ion exchange, but thereare important differences. In the chemical tempering process theconcentration profile and compressive stress profile build upsimultaneously as ion exchange occurs. Unfortunately this drasticallylimits the thickness of strengthened surface layers due to the fact thatthe rate of surface stress decay by viscous flow eventually exceeds therate of stress buildup by diffusion. This limits the thickness of thecompressive stress region, causing chemically tempered ceramics to besusceptible to failure owing to scratching. In molecular stuffing theconcentration profile and surface stress are introduced in separatesteps, so that there is no barrier to the production of thick surfacelayers. This allows the glass to retain its strength even when itsustains deep scratches.

Using the present invention, it is possible to control the appearance inaddition to increasing the strength of a glass article. For example, ifdopants are added which do not readily dissolve at consolidationtemperatures (described below) they will give a milky or opalescentappearance to the sample. Such dopants are refractory oxides such as theoxides of aluminum, the alkaline earths, titanium, chromium, zirconiumand zinc.

If color is desired, a transition metal or rare earth ion can be used.For example, under oxidizing conditions copper and iron will make thesample blue and brown or yellow respectively; under reducing conditionscopper and iron will make the sample red and green respectively.Generally cobalt will make it blue, and neodynium, purple.

To obtain a smoky appearance one could use any element which contributesfree electrons. Examples of these are germanium, tin, lead and bismuth,all in a reducing atmosphere. In addition the presence of colloidalmetallic precipitates of the noble metals will create various smokytints.

In any process for forming the glass articles of this invention, theformation of the porous matrix constitutes a value added step and losses(for example, breakage) after this stage may decrease the economic yieldof a full-scale commercial process. We have found it difficult todetermine the factors causing losses either through breakage due tocracking or the occurrence of light scattering centers such asinclusions or bubbles in the final collapsed article.

To obtain both increased yields and more control over refractive indexprofile consistently and satisfactorily, we have now and improved thosestages of the process where it is necessary to operate in a particularmanner, and in a particular sequence not previously disclosed.

With regard to yield, as in the normal manufacture of any glass articlemodifications to the process do not necessarily result in every articlecast or formed being fault free and ensure that such articles will allsurvive the subsequent processing steps. By an increased yield we meanthat we have found how to improve the statistical chances of a rod orother article surviving the processing steps. This is, however, on astatistical basis and one cannot guarantee that even when all theessential steps of our process are used, an acceptable product willalways be obtained. As indicated previously, the improvement produced isnot only in yield but in insuring that a desired strengthening profileis obtained consistently and satisfactorily.

This is primarily based on our discovery that for best results it isessential that the step of depositing the solid material in the pores becarried out by a process which does not involve evaporation of solvent,and that the subsequent heating step to raise the temperature of thearticle so as to remove the solvent from the pores, and, wherenecessary, decomposition products should be regulated so as to achieveretention or production of a desired strengthening profile.

We have also found that while achieving a satisfactory article, certaindopants give particularly advantageous results because of their physicalcharacteristics.

We also prefer that, if the porous article to be stuffed has been madeby phase separation from a glass followed by leaching, certainprecautions be taken to reduce losses during the processing of the rod.We have found that cracking in this form of our process can be caused byproblems arising from one or more of the following:

(a) Incorrect glass composition;

(b) Incorrect heat treatment conditions for phase-separation; and

(c) Incorrect leaching procedure.

Guidance is given below as to how to choose glass composition andprocessing conditions so as to reduce loss due to cracking in subsequentprocessing both during the formation of the porous matrix and thesubsequent stuffing and drying.

The present invention is concerned with a method of producing a desiredstrengthening profile in a glass article as a function of its dimensionsby the addition of a strength modifying component (hereinafter referredto as a dopant) to a porous matrix with interconnective pores whosewalls are formed from at least one glass network forming component and,where desired, glass network modifying components. The method comprisesthe steps of immersing the porous matrix in a solution of a dopant,causing the dopant to separate in the matrix, removing solvent and,where necessary, decomposition products from the porous matrix andcollapsing the porous matrix to a solid form, characterized in that partor all of the dopant is caused to be precipitated by a method which doesnot involve evaporation of solvent, the removal of solvent is notcommenced until a substantial part of the precipitation has taken placeand the rate at which heat is applied to remove solvent, and wherenecessary, decomposition products is regulated so as to achieve and/orretain the desired strengthening profile within the glass article.

The steps and the sequence of steps which we have found suitable toproduce a particular profile are outlined below, all starting with aporous article having interconnected pores.

The steps of our invention comprise the following:

(1) The dopant is precipitated in the pores by non-evaporative steps.These include (a) thermal precipitation in which by lowering thetemperature of the object, the solubility of the dopant or dopantcompound in the solvent is decreased sufficiently to cause precipitationof the dopant or dopant compound and (b) chemical precipitation such asalteration of solution pH to a point of precipitation, replacement ofthe original solvent by a solvent in which the dopant or dopant compoundis less soluble or introduction of a chemical into the solution whichreacts with original dopant or dopant compound to form a less solubledopant species. Hereinafter the term solvent is used to describe thechemical species which at some stage is the liquid filling the pores.

(2) Removal of the final solvent is commenced only after precipitationis substantially complete.

(3) The rate at which heat is applied to remove solvent and wherenecessary, decomposition products, is regulated so as to achieve and/orretain the desired strengthening profile within the glass article.

The steps and the sequence of steps which we have found suitable toproduce stepped profile (see FIG. 4a) and continuous profile (see FIG.4b) are outlined below, starting with a porous article and using thermalprecipitation of dopant or dopant compound.

Stepped profile

(1)

(a) Immerse the porous matrix in a solution of dopant or dopantcompound.

(b) Precipitate the dopant by dropping the temperature.

(c) Immerse in a solvent for the dopant and allow the dopant topartially redissolve and diffuse out of the matrix. Only the dopantprecipitated near the outer surface of the article is removed in thisstep.

(d) Evaporate any solvent.

(e) Heat to collapsing temperature.

(2) Alternatively:

(a) Immerse the porous matrix in a solution of dopant or dopantcompound.

(b) Precipitate the dopant by dropping the temperature.

(c) Partially dry the porous rod.

(d) Immerse in a solvent for the dopant and allow the dopant topartially redissolve and diffuse out of the matrix. Only the dopantprecipitated near the outer surface of the article is removed in thisstep.

(e) Evaporate any remaining solvent.

(f) Heat to collapsing temperature.

Continuous profile

(a) Immerse the porous matrix in a solution of dopant or dopantcompound.

(b) Immerse in a solvent for the dopant at substantially the sametemperature as that at which stuffing took place. The article remains inthe solvent for a precalculated time.

(c) By repeating step B with baths of varying dopant concentration, thediffusion time history can be controlled to create the desired profileof dopant in the article.

(d) Precipitate dopant in pores by dropping temperature.

(e) Evaporate the solvent.

(f) Heat to collapsing temperature.

As indicated above, we prefer to use as a dopant a material whosesolubility characteristics are such that we can achieve the desiredconcentration of the dopant in the pores, by diffusing a solution of thedopant into the pores at one temperature and then cause itsprecipitation by a simple drop in temperature. We refer to such aprocess as thermal precipitation. While we prefer to use this process,other routes are feasible. Our invention therefore includes a processfor the production of a glass article with a desired strengtheningprofile using a suitable porous matrix as a starting material in which astrength modifying component is caused to separate out of solution bylowering the temperature of the solution.

Amongst other routes we find we can precipitate the solute by chemicalmeans rather than by temperature drop. The common ion effect has beenused to reduce solubilities and cause precipitation of the solute (e.g.,the solubility of CsNO₃ in water is reduced in the presence of 1 NHNO₃). The exchange of solvents has also been used to reducesolubilities and thus precipitation by means not involving evaporationof solvent can be used. These include the addition of a suitableprecipitant which reacts with the dopant or cause a suitable change inpH. We have also used a combination of steps consisting of both thermaland chemical precipitation means. This is particularly useful in casesin which more than one dopant or dopant compound is being introducedinto the pores. We avoid any precipitation methods involving evaporationof solvent as the sole means of precipitation, since we have been unableto obtain consistent results using such methods. We believe this is dueto the following factors.

In the direct evaporative process the solution evaporates from thesurface of the article causing transport of the dopant from the interiorto the surface. There is also a vertical transport process due togravity which causes accumulation of the dopant at the bottom of thearticle. Together these effects tend to produce undesirable profiles.

It is essential to regulate the rate of heating so as to avoiddestroying either the incipient strengthening profile, or damaging theinterconnective pore structure. It is possible by allowing the evolutionof vapor or gas in an uncontrolled manner to produce a pressuresufficient to destroy the integrity of the structure. We prefertherefore to avoid allowing the solvent to reach its boiling point at apoint when large volumes of vapor are liable to be produced. Variousheating regimes are described below, and show how to regulate theheating to achieve the desired end.

The regulation of the solvent removal step is based on the need to avoiddestruction of the integrity of the porous structure, and upsetting thedistribution of the dopant in the pores. The precautions we take are tocommence solvent removal at room temperature or below by a non-boilingmethod, and avoid any violent change in temperature which would cause anexcessively fast evolution of solvent vapor in a confined space.Convenient methods of commencing solvent removal include placing thearticle, where the solvent is water, in a dessicator at about 22° C. forabout 24 hours, or in a vacuum at temperatures slightly above 0° C.(i.e., 4° C.) for about 4 hours, and then to proceed to raise thetemperature. We have also found that it is necessary in some cases tohold or reduce the heating rate to a very low value so that the articlestays in a particular temperature range for a time sufficient to ensureparticular events have occurred before heating is continued. At otherpoints we believe it preferable to move rapidly from one temperature toanother, e.g., when solvent removal has been completed to thetemperature of collapse. Later in this specification we give someguidance in terms of an aqueous system, but the warnings given can beseen to apply equally to the system where organic solvents or othernon-aqueous systems or mixtures of such systems are used.

The following criteria can be used to select a suitable dopant fromamong the larger group of strength modifying components.

(a) It must be soluble in suitable concentrations in a solvent whichdoes not interfere with subsequent processing after stuffing.

(b) It must be able to be incorporated into the matrix either asdeposited or after thermally induced decomposition.

(c) the dopant must not change its physical or chemical state in such away before collapse as to be lost from the matrix.

(d) For high strength, the following added conditions apply:

(1) When using T_(g) method to create strengthening profile, the dopantmust decrease the glass transition temperature of the porous matrix.

(2) When using the expansion coefficient method to create strengtheningprofile, the dopant must increase the expansion coefficient of theporous matrix.

(3) When using the mixed method to create strengthening profile, thedopant must decrease T_(g) and must increase expansion coefficientsimultaneously.

In this invention, since high light transmission is not a necessity,compounds of most elements can be used as dopants. For example, tocreate a T_(g) profile the choice is not critical. Two useful dopantsare Na₂ O and B₂ O₃ which lower the T_(g) of the matrix by 80° C. and40° C. respectively for each percent of dopant. Others which can be usedto create a T_(g) -profile include but are not limited to oxides orsalts, such as chlorides, silicates, borates, phosphates, and germanatesof lithium, potassium, boron, phosphorous, germanium and arsenic.

There are many dopants which can be used to increase the thermalexpansion of the matrix. For example, cesium oxide raises the expansioncoefficient. Other dopants which can be used include but are not limitedto the oxides or salts of the heavy elements such as barium, lead,thallium, bismuth, thorium and uranium. In addition, oxides or salts ofrubidium, strontium, antimony and tin may be used.

Other dopants and mixtures of dopants can be used as long as the abovecriteria are satisfied. It is impossible to list all the potentialcombinations of dopant elements but it is believed that based on theguidance given, such selection of useful combinations is within thecompetence of those practiced in the art.

In the selection of solvents the following considerations are important.The solvent selected

(a) should not damage the porous matrix;

(b) should be one that can be substantially removed by either exchangewith another solvent, evaporation, or thermal decomposition followed byoxidation (or high temperature reaction with chemically activeatmosphere);

(c) should have sufficient solubility for the dopant compound orcombination of dopant compounds to allow the desired dopant levelswithin the pores to be achieved by molecular stuffing.

(d) should be such that, if used in thermal precipitation process, anydopant solutions in the solvent will have sufficiently high temperaturedependence of solubility to deposit dopant within the pores when cooled;

(e) should be such that, when used to precipitate by a solvent exchangeprocess, will have the specific solubility properties as needed by theprocess;

(f) should be, for economic considerations, low cost and capable of highspeed of drying.

It is impossible to test all possible combinations of solvents; howeverwe have found that water, alcohols, ketones, ethers, mixtures of theseand salt solutions in these solvents can be used satisfactorily applyingthe above criteria to the selection of a particular solvent.

In general, we prefer to use thermal precipitation because of its easeand convenience, and because we prefer to carry out the first stage ofthe subsequent solvent removal step after stuffing at room temperatureor below, and hence it is usually necessary to cool the stuffed article.

The dopants used are preferably water soluble and have a steepsolubility coefficient, that is, that the material is very soluble attemperatures of the order of 100° C., and on cooling to room temperatureor below, a substantial amount of material separates, thus making themsuitable for thermal precipitation.

Further detailed guidance concerning the choice of solvent is given byreference to Table I below in which the solubility of various dopants insolvents at different temperatures are illustrated.

As already indicated above, in choosing a particular route to a desiredend product, a number of guidelines need to be considered, and these canbe illustrated by reference to Table I.

First, in order to obtain the desired concentration of dopant in thearticle to yield a significant increase in strength, a solution having asufficiently high concentration of dopant must be found by suitablechoice of dopant compound, solvent and temperature. In order toprecipitate the dopant or dopant compound, the use of solvents withsufficiently low solubility is necessary. Often there is a need toremove substantial amounts of dopant from designated areas of thearticle, such as in the surface region of a glass article, in which casesolvents with intermediate solubilities are useful. Such removal ofdopant is referred to as unstuffing, as opposed to molecular stuffing.Suitable control of solubilities for proper precipitation of the dopantor dopant compound can be achieved by a number of methods.

(1) Thermal precipitation is most suitable for solvents whose solubilityfor the dopant or dopant compound is strongly temperature dependent.Thermal precipitation has the added advantage of being able to arrestdiffusion in the shortest time, thus enabling us to freeze-in a desiredcomposition profile with high accuracy.

This is illustrated for CsNO₃ dopant in Table I whereby the solubilitychanges from a desirable stuffing level at 95° C. to a desirableunstuffing level at 4° C.

(2) Precipitation by common ion effect and thermal precipitation.Precipitation can be produced or further enhanced by the common ioneffect. For example when the dopant is a nitrate the concentration ofnitrate ions in solution is increased by adding another source ofnitrate ions to the solvent (i.e., HNO₃ acid). This reduces thesolubility of the nitrate dopant (see CsNO₃, Table 1)

(3) Precipitation by solvent exchange. Precipitation is induced bysubstituting a low solubility solvent for a higher solubility solvent.The high solubility of nitrates in water has allowed us to use water assolvent for the stuffing process. Exchange of water with eitheralcohols, ketones or ethers or combinations has induced precipitation ofthe dopants. Typical solubilities are illustrated in Table I.

(4) Variation in dopant compound. The range of solubility of the dopantcompounds may be altered by choosing a different anion such as replacingCsNO₃ by Cs₂ (CO₃) to increase solubility in water (see Table I, line7).

                                      TABLE I                                     __________________________________________________________________________            Solubility metha-                                                                            etha-                                                                            1-pro-                                                                            diethyl  Temp.                                  Compound                                                                              gr/100 ml sol.                                                                       H.sub.2 O                                                                         nol nol                                                                              panol                                                                             ether                                                                             HNO.sub.3                                                                          (°C.)                           __________________________________________________________________________      CsNO.sub.3                                                                          >100   100%                    >95                                      "     110    100%                    4                                        "     4                         1 Normal                                                                           4                                        "     1       30%                                                                              70%                 4                                        CsNO.sub.3                                                                          10      70%                                                                              30%                 22                                       "     1       15%                                                                              85%                 22                                       Cs.sub.2 CO.sub.3                                                                   >100   100%                    22                                       "     10         95%     5%          22                                       "     1          52%    48%          22                                     10.                                                                             Pb(NO.sub.3).sub.2                                                                  >100   100%                    >70                                      "     10      30%                                                                              70%                 22                                       "     1       5% 95%                 22                                       La(NO.sub.3).sub.3                                                                  >100   100%                    22                                       "     10             15%    85%      22                                       "     1              10%    90%      22                                       Nd(NO.sub.3).sub.2                                                                  20             10%    90%      22                                       "     1               7%    93%      22                                       Ba(NO.sub.3).sub.2                                                                  32     100%                    95                                       "     10     100%                    22                                     20.                                                                             "     5      100%                    0                                        Al(NO.sub.3).sub.3                                                                  63.7   100%                    22                                       H.sub.3 BO.sub.3                                                                    27.6   100%                    100                                      "     6.35   100%                    22                                     __________________________________________________________________________

As indicated above, when operating the process of the present inventionwith a porous interconnective structure which has been produced by thephase separation of a suitable glass followed by a leaching step, it isnecessary to optimize the various stages of the process in order toachieve consistently and satisfactorily a saleable end product in goodeconomic yields, and to interrelate the various parameters involved.

The factors on which guidance is required by the man practiced in theart are:

(1) selection of glass composition and heat treatment to obtain suitablephase separation;

(2) leaching and washing;

(3) stuffing;

(4) unstuffing where needed; and

(5) drying and consolidation.

The guidance given in steps (3-5) above applies to all matrices, notjust those produced from a phase separated glass.

1. Glass composition, time and temperature of heat treatment.

In order to achieve a satisfactory product it is necessary to choose aphase-separable composition, which on heat treatment at a particulartemperature separates into approximately equal volume fractions, andwhen held at that temperature, develops an interconnective structurewith a desirable pore size. A number of guidelines can be given to theman practiced in the art. We find it convenient to choose compositionsfrom the area of alkali metal borosilicate glasses, and further guidanceis given below as to suitable compositions.

Many compositions have been reported as suitable for use in theproduction of porous glasses for diverse purposes (see U.S. Pat. Nos.2,272,342, 2,346,059, 3,859,073 and 3,831,640) usually not forstrengthening purpose by a route based on phase separation and leachingof the soluble phase. We have discovered that for strengthening purpose,only small regions within prior art compositions ranges are suitable.U.S. Pat. No. 3,843,341 is one representative disclosure of suchcompositions which for the most part are not satisfactory. For example,a number of glasses from within the preferred region of U.S. Pat. No.3,843,341 and from previous disclosures of Corning (see Table II below)were vertically drawn into 8 mm rods at a rate of one inch per minute.These were phase separated as disclosed herein, but all the Table IIcompositions cracked during leaching.

                  TABLE II                                                        ______________________________________                                        Prior Art Compositions which Crack upon Leaching*                             Na.sub.2 O   B.sub.2 O.sub.3                                                                         SiO.sub.2 Al.sub.2 O.sub.3                             ______________________________________                                         8           30        62        0                                             8           35        57        0                                             8           40        52        0                                            10           30        60        0                                            10           35        55        0                                            10           40        50        0                                            10           30        59        1                                            10           35        54        1                                            10           40        49        1                                            ______________________________________                                         *All concentrations are in units of mol percent.                         

More specifically, we have found that

(1) All of the compositions in the range of the sodium borosilicatesystem disclosed in U.S. Pat. No. 3,843,341 and drawn into rods asdescribed above, cracked upon leaching. This includes the region denotedas the preferred range in said patent.

(2) Many of the compositions in the range of the sodium aluminaborosilicate disclosed in U.S. Pat. No. 3,843,341 cracked when treatedas described above. This was true even for compositions in the preferredrange.

(3) Many of the preferred compositions disclosed in the U.S. Pat. No.2,221,709 cracked when treated as described above.

Although we find most of the previously disclosed borosilicate range ofcomposition unsatisfactory because of the requirements we need to insurea satisfactory yield of product, we have discovered certain specificcompositions in this broad range which are useful and which have notbeen previously described. In addition we have discovered a set ofcriteria which can be applied to identify other specific limited areasof phase-separable glass compositions which would give a satisfactoryyield of product.

From a commercial point of view, and because of the large region ofphase separation which they show it is most convenient to work withalkali borosilicate glasses though almost all silicate glassy systemsexhibit composition regions of phase separation.

In order to achieve a satisfactory product it is necessary to choose acomposition:

(1) which on suitable heat treatment separates into two phases, onesilica-rich, the other silica-poor, the latter is preferentially solublein a suitable solvent;

(2) which on heat treatment at a particular temperature separates intophases of approximately equal volume fractions and when held at thattemperature develops interconnected microstructure;

(3) which is easy to melt and is easy to refine using conventionaltechniques;

and (4) which is relatively easy to form and does not phase separatesignificantly during the forming stages.

The following provides a systematic procedure for selecting a suitablecomposition.

(1) Almost all silicate glassy systems exhibit composition regions ofphase separation. However, of commercial interest are the alkaliborosilicate glasses which show a large region of phase separation. Thesilica-poor phase of these glasses can be readily dissolved by simpleacidic solutions. Frequently it is necessary to add other componentssuch as aluminum oxide to modify certain properties of these glasses.However, some oxides are not desirable because on phase separation theyend up in the silica-poor phase and make it difficult to dissolve bysimple acidic solutions. Thus only those other components are suitablewhich do not diminish the solubility of silica-poor phase significantly.

(2) After deciding on the glassy system (along with dopants), one shouldnext determine the immiscible regions of composition, C, and theircoexistence temperatures, Tp. Tp(C) is the temperature above which aglass (C) is homogeneous. Techniques for determining Tp are welldescribed in literature (see for example W. Haller, D. H. Blackburn, F.E. Wagstaff and R. J. Charles, "Metastable immiscibility surface in thesystem Na₂ O-B₂ O₃ -SiO₂ " J. Amer. Ceram. Soc. 53 (1), 34-9 (1970)).

(3) We have discovered that one should determine those compositions, C₁,which exhibit an equilibrium volume fraction of about 50% at least atsome heat treatment temperature. We shall denote this temperature byTo(C₁). The method for determination of this temperature is as follows:

(a) Select three (or more) temperatures, say T₁, T₂ and T₃ (about 50°apart from each other) such that

T₃ <T₂ <T₁ <Tp(C₁).

Carry out long heat treatments on glass samples until they turn white attemperatures T₁, T₂ and T₃.

(b) By electron microscopy of these heat-treated samples, measure volumefractions, V(T), of one of the phases (the same phase should be selectedfor all samples) for each of the samples.

(c) Make a plot of volume fraction V(T) against heat treatmenttemperature, T. By interpolation (or extrapolation) determine thetemperature for which the volume fraction will be 50 (±5)% (i.e.,temperature To(C₁)).

(4) Knowing Tp(C) and To(C₁), one should determine the composition range(C₂) within the range C₁ such that

(a) 575° C.≧To (C₂)≧500° C. and

(b) 750° C.≧Tp (C₂)≧600° C.

These temperatures are selected so as to give not over long heattreatment times; other ranges can of course be selected if one iswilling to accept long heat treatment times of a week or more.

(5) The composition range, C₂, is further narrowed by the requirementthat, during melting, it should be easily refinable. This demands thatthe high temperature viscosity of the melt should be sufficiently low.We shall call this sub-range of C₂, C₃ (i.e., all glasses belonging toC₂ which, in addition, refine properly). We have found for example thatone convenient feature identifying some of the refinable glasses is thatthose containing at least 28% B₂ O₃ refine satisfactorily.

(6) Not all compositions of C₃ are desirable even though they willrefine easily, and will phase separate with 50% volume fraction. Anadditional requirement is that the desired composition should not phaseseparate significantly during forming operation. The degree of phaseseparation in the forming process is influenced by the viscositycharacteristics of the glass in the region at and below the co-existencetemperature, and also the dimensions of the article being formed, andrate at which it can be cooled.

In order to determine those compositions, C₄, which do not appreciablyphase separate upon cooling through and below the co-existencetemperature, articles with wall thickness of the order of several mm arecooled at rates sufficiently low to prevent the build-up of largethermal stresses. The degree of phase separation occurring within thesearticles can then be determined. Those compositions within the area C₃which do not phase separate in this forming process can then be groupedin the further restricted area C₄. The compositions fulfilling thiscondition preferably have a Tp between 710° and 600° C., most preferablybetween 695° and 640° C.

(7) The final criterion we apply insures that there is sufficientcomposition difference between the two phases when separated thatleaching will take place effectively. For this purpose we select onlythose compositions C* which within the C₄ range satisfy the conditionthat

Tp(C*)-To(C*)≧75° C.

Having found the desired composition range C*, any composition, Co, canbe selected. The suitable heat treatment temperature and time for thisparticular composition Co can then be found by the following procedure.

(a) The heat treatment temperature of Co is set equal to To(Co), i.e.,the temperature at which the volume fraction of the two phases will beequal. If C* has been properly chosen according to the above criteria,this temperature will not be so low that the time needed to obtain asuitable microstructure for leaching would be too long and uneconomic.Similarly, it will not be too high otherwise

[1] distortion of the glass may occur during heat treatment; and

[2] if the temperature of heat treatment is well above say, over 160centigrade degrees above the glass transition temperature, phaseseparation tends to be rapid which reduces the degree of control onphase separation. These requirements limit our preferred heat treatmenttemperature, T_(H), to the following range

575° C.>T_(H) (Co)=To(Co)>500° C.

(b) Having found T_(H) (Co), the heat treatment time is determined bythe condition that a microstructure state suitable for leaching isdeveloped.

Heat treatments at different times are carried out say t₁ (1 hour)<t₂ (2hours)<t₃ (3 hours) . . . By electron microscopy, it is possible todetermine the time, t_(max), beyond which the interconnectivity of thestructure begins to break down. The size of the leachable phase ismeasured from micrographs, and the preferred heat treatment times arethose which are less than t_(max) but for which the microstructure sizeis at least 150 A, and preferably less than 300 A.

We have applied the above criteria within the alkali borosilicate systemand have identified certain characteristics of the composition rangeswhich contribute to good yields of end products and particularly thosehaving appreciable thickness, avoiding the problems arising from, e.g.,phase separation during forming, or insufficient phase separation whenthe phase separation stage is being carried out. Phase separation duringforming of the glass article from the melt, and insufficient phaseseparation or breakdown of the interconnective structure during thephase separation heat treatment can both cause or contribute to crackingduring either or both of the following steps, leaching of the phaseseparated glass, drying of the leached and stuffed glass.

It has become clear to us that the compositions associated with the bestyields are those contained within the following broad composition area(all percentages being in mol percent):

    ______________________________________                                                        Broad       Preferred                                         ______________________________________                                        SiO.sub.2         48-64         49.5-59                                       B.sub.2 O.sub.3   28-42         33-37                                         R.sub.2 O         4-9           6.5-8                                         Al.sub.2 O.sub.3  0-3           0-2.0                                         ρ             0-1.0         0.20-0.8                                      α           0-3           0-2.4                                         λ          0-0.5         0                                             x                 0.1-1.0       0.2-0.8                                       ______________________________________                                    

where α is the Al₂ O₃ concentration in mole percent, x=ρ+(1/3)α-λ and ρis defined as the ratio A₂ O/R₂ O for A₂ O and R₂ O in mole percent, A₂O is the sum of the concentrations in mole percent of K₂ O, Rb₂ O andCs₂ O; and R₂ O is the sum of the concentrations in mole percent of Li₂O, Na₂ O, K₂ O, Rb₂ O, and Cs₂ O; and λ is the ratio Li₂ O/R₂ O.

Because of the presence of Al₂ O₃ in the glass significantly affects theresults, we will first discuss glasses which have no Al₂ O₃ content.Under these conditions, the ranges listed above are appropriate with Al₂O₃ content of zero, with R₂ O the sum of all the alkali metal oxides Li₂O, Na₂ O, K₂ O, Rb₂ O Cs₂ O and the broad range for ρ limited between0.1 and 1.0. If the concentration of K₂ O is zero, then the upper limitof the range for ρ should be 0.8.

Lithium glasses tend to devitrify and therefore it is often preferablenot to use that chemical. In this case, R₂ O becomes the sum of theconcentrations of Na₂ O, K₂ O, Rb₂ O and Cs₂ O. All limits andconditions above are maintained.

Rubidium and cesium glasses are more expensive than those made withsodium and potassium. They can be left out for economic reasons. Then R₂O becomes the sum of Na₂ O and K₂ O. All limits and conditions above aremaintained.

When more than 0.5 mole percent Al₂ O₃ is present in the glass, thebroad range of R₂ O is taken between 6 and 9 mole percent.

The most economically favorable compositions with Al₂ O₃ consist of R₂ Ohaving Na₂ O and K₂ O only, or R₂ O can also consist of Na₂ O only.

The glasses below in Table III are glasses which we have identifiedusing the above criteria and found satisfactory for use in the molecularstuffing process of the present invention as we achieve a satisfactorycontrol of phase separation and pore structure after leaching usingthese compositions, and a good overall yield of finished product of theinvention.

                                      TABLE III                                   __________________________________________________________________________    Leachable Compositions                                                        SiO.sub.2                                                                           B.sub.2 O.sub.3                                                                  Na.sub.2 O                                                                        K.sub.2 O                                                                        Rb.sub.2 O                                                                        Cs.sub.2 O                                                                       Al.sub.2 O.sub.3                                                                  ρ                                                                            α                                                                          λ                                                                        x  Tp                                      __________________________________________________________________________    I  63 30 5   2  0   0  0   0.29                                                                             0  0 0.29                                                                             --                                      II 63, 7                                                                            29.4                                                                             6.3 0.6                                                                              0   0  0   0.10                                                                             0  0 0.10                                                                             --                                      III                                                                              61.2                                                                             34 2   2.8                                                                              0   0  0   0.58                                                                             0  0 0.58                                                                             646                                     IV 60.7                                                                             35.1                                                                             2   2.2                                                                              0   0  0   0.52                                                                             0  0 0.52                                                                             --                                      V  59.7                                                                             33 5.4 1.9                                                                              0   0  0   0.26                                                                             0  0 0.26                                                                             678                                     VI 59.3                                                                             34.3                                                                             1.6 4.8                                                                              0   0  0   0.75                                                                             0  0 .75                                                                              633                                     VII                                                                              58.4                                                                             34 5.6 0  2   0  0   0.26                                                                             0  0 0.26                                                                             687                                     VIII                                                                             59.1                                                                             34 4.9 0  0   2  0   0.29                                                                             0  0 0.29                                                                             687                                     IX 57.7                                                                             35 5.7 0  0   0  1.6 0  1.6                                                                              0 0.53                                                                             --                                      X  56 36 4   4  0   0  0   0.50                                                                             0  0 .50                                                                              670                                     XI 56 36 6   2  0   0  0   .25                                                                              0  0 .25                                                                              710                                     XII                                                                              53 38 8   0  0   0  1.0 0  1.0                                                                              0 .33                                                                              681                                     __________________________________________________________________________

Another aspect of our invention involves leaching of borosilicate phaseseparable glasses. We prefer before leaching the glass to etch thearticle to be leached with dilute hydrofluoric acid for about 10 secondsto remove any surface contamination, or surface layer of glass having aslightly different composition from the interior due to volatilizationof components such as B₂ O₃ or Na₂ O during formation.

The concentration of the acid solution, amount of leaching solution andtemperature of leaching have a direct bearing on the progress ofleaching. It is essential to insure that a sufficient quantity of theleaching solution is brought into contact with the article to dissolvethe soluble phase. The rate of leaching may be conveniently controlledby adjusting the temperature. The glass should be above 80° C.,preferably above 90° C. As has previously been described in U.K. Pat.No. 442,526, it is desirable to use an acid solution which has beensaturated with NH₄ Cl or other equivalent compounds capable of reducingthe concentration of water in the acid leaching solution. This assistsin controlling any swelling of the treated layer and reducesconsiderably the chances of loss due to cracking of the article, as theinner untreated layer goes into tension because of the thickness of theswollen outer layer.

We have found that the rate of leaching, and the redeposition of boratesin the pores of the glass during leaching can be controlled bycontrolling the concentration of borate salts in the acid leachingsolution.

We have measured leaching rates at 95° C. for glass rods (length 10 cm,diameter 8 mm, and composition 57% SiO₂, 35% B₂ O₃, 4% Na₂ O and 4% K₂O) heat treated for 11/2 hours at 550° C. with leaching solutionscontaining 327.3 gm of NH₄ Cl, 33.6 ml of HCl per liter of water andvarying amounts of B₂ O₃. We found that leaching time increased withincreasing B₂ O₃ content in the leaching solution. The results aresummarized below:

                  TABLE IV                                                        ______________________________________                                        Amount of Boric Acid (g/liter)                                                                   Leaching Time(minutes)                                     ______________________________________                                        0                  125 ± 50                                                41.2               625 ± 50                                                61.5               542 ± 50                                                84.7               725 ± 50                                                106.1              1670 ± 50                                               ______________________________________                                    

We believe redeposition of borates in the pores also contributes tobreakage. This can be avoided by e.g. replacing the leaching solution asthe concentration of borate builds up. But this requires largequantities of leaching solution. For example, in order for leaching timeto be no more than 660 minutes, the volume of leaching solution per 100ml of glass will be on the order of 1550 ml. This, however, can increasecosts and provide a possible source of contamination. We find it moreconvenient to provide a cold trap so that excess material iscontinuously removed from the solution as it comes into solution fromthe article being leached. The cold trap is effective in speeding theprocess even if it is only a few degrees below the temperature of theglass article.

Preferably it should be 20° C. below the temperature of the glassarticle. We find it convenient when NH₄ Cl is present to choose atemperature for the cold trap at which the acid solution remainssaturated with NH₄ Cl. It is possible to operate with a low level of rodbreakage without NH₄ Cl or other equivalent compounds present in theleaching solution. In general we prefer to use at least 10 weightpercent NH₄ Cl, preferably 20 weight percent as we find that on astatistical basis there is an even lower level of rod breakage when NH₄Cl is present.

The most convenient way to determine a suitable leaching time is to takean article and subject it to the leaching treatment measuring the massof the article at intervals of time until little or no further weightloss is observed.

The article, once leached, is conveniently washed with deionized water.With certain compositions there can be deposition of silica gel in thepores, and we find this can be removed by washing with NaOH. We havefound it possible by selection of compositions to minimize thisdeposition. The compositions shown in Table III alleviate this problem,especially those with minimum silica.

Once the porous matrix has been produced, either from a phase-separableglass as outlined above, or by, e.g., a chemical vapor depositiontechnique, the selection of suitable conditions for stuffing andunstuffing we have found can be made by following the guidelines givenbelow.

The dependence of T_(g) or expansion coefficient on dopant and dopantcompound concentration can be determined by literature search or bysuitable experiments. From these, an optimum concentration of dopant ordopant compound needed in the article is determined. Sufficient dopantor dopand compound must then be dissolved in the stuffing solution sothat the desired concentration is reached at a particular stuffingtemperature and time of stuffing. The following procedure enables theseparameters to be determined:

(1) Determination of Stuffing Temperature of Porous Articles

(a) Determine the dependence of the solubility of the dopant or dopantcompound in the appropriate solvent on temperature.

(b) The stuffing temperature range lies between the temperature at whichthe desired concentration of dopant or dopant compound is saturated insolution from (a) and the boiling temperature of the solution.

(2) Determination of Stuffing Time of Porous Articles

The stuffing time depends not only on the concentration, temperature,and the composition of dopant solution, but also on the microstructuresize in the porous article. The procedure given here is for a given setof dopant solution, temperature and microstructure of rod shapedarticles. For a change in any of these variables, the procedure shouldbe repeated, or suitably modified according to our guidelines.

(a) Measure the diameter (a_(o)) of a porous rod and immerse it in thedopant solution.

(b) Monitor its weight as a function of time.

(c) Determine the time, t_(o), beyond which the weight does not increasesignificantly by plotting the fractional weight change,y(t)=[M(t)-M(o)]/[M(∞)-M(o)] versus time, t, where M(o), M(t), and M(∞)are the respective weights initially, at time t and at infinity (verylong times).

(d) Time required to stuff, t, another porous rod of diameter a, withthe same dopant solution at the same temperature is

    t=t.sub.o [a/a.sub.o ].sup.2.

EXAMPLE

We stuffed a porous rod with a concentrated solution of CsNO₃ in water(120 g CsNO₃ per 100 ml of solution) at 100° C. The radius of the rodwas 0.42 cm. We measured the weight gain as a function of time. Theresults are shown in FIG. 6. It can be seen that after about 200 minutesthe weight of the rod does not increase significantly. Thus, the properstuffing time for this rod is about four hours.

(3) Determination of Unstuffing Time to Produce a Continuous Profile ina Porous Article by Thermal Precipitation

To produce a continuous profile, the stuffed article, produced as (2)above, is partially unstuffed by immersing it in the solvent. Thisshould be done at a temperature where the dopant does not precipitate.The unstuffing procedure depends on the temperature, microstructure andcomposition profile desired. For example, the procedure described hereis for a given temperature of stuffing and parabolic profile.

(a) Carry out an unstuffing study at the temperature at which the rodwas stuffed by monitoring the weight change as a function of time whilethe rod is immersed in solvent.

(b) Plot the fractional change y(t) ##EQU1## against time t.

(c) The time of unstuffing, t_(o), depends on the desired profile; oftenit is

1/3≦y(t_(o))≦2/3.

For profiles of other shapes, a similar formula for y(t_(o)) can beworked out.

EXAMPLE

We chose a porous rod stuffed with concentrated CsNO₃ solution (120 gCsNO₃ per 100 ml solution) at 100° C. as described above. We thenunstuffed the rod in water at 100° C. monitoring its weight loss as afunction of time. The results are shown in FIG. 6. The range ofunstuffing times can be calculated from the graph.

(4) Determination of Unstuffing Temperature and Time to Produce a StepProfile by Thermal Precipitation

The temperature for unstuffing for a step profile depends on thestrength desired in the article and on the dopant solution. Since onewould like to have as low an expansion coefficient (or high T_(g)) inthe surface layer as possible, the unstuffing temperature is typically afew degrees above the freezing point of the dopant solution.

The time required for unstuffing depends on the desired thickness of thesurface layer to be put in compression, as well as on such parameters astemperature of unstuffing, the concentration of the stuffing solutionpreviously used, and the size of microstructure in the porous article.The procedure described here is for a given set of these variables. Incase of a change in the values of any of these parameters, the entireprocedure should be repeated, and adjusted according to the guidelinesherein.

Suppose the desired surface layer thickness is "d" and the thickness ofthe article wall is "a". Let

    Y=4(d/a)[1-(d/a)g],

where g is a geometric factor, g=1 for cylindrical shapes; g=0 forplates. Knowing Y, it is possible to determine the proper unstuffingtime by following the prodedure described below.

(a) Carry out an unstuffing study in the desired solution at the desiredtemperature by monitoring the fractional weight change y(t) as afunction of time.

(b) Plot the fractional weight change against time, (see Eq. 1).

(c) Find time t_(o) for which

    y(t.sub.o)=Y

from the above plot. This is the desired unstuffing time.

The practical application of the use of the unstuffing procedure is inmost cases to reduce the concentration of dopant in the outer layers ofthe article so as to attain a desired strengthening profile.

This can be done as indicated above by, e.g., when the actual stuffinghas been completed with a saturated solution of a dopant at 95° C.,replacing the dopant solution by the solvent used free of dopant at thesame temperature, or where the system is aqueous, water or dilute nitricacid. The dopant then tends to diffuse outwards, thus varying theconcentration through the cross-sectional area of the porous matrix. Thetime required for this "unstuffing" is of course dependent upon thevolume being treated, but an article of wall thickness 8 mm requiresabout 20-30 min. We prefer to stop the unstuffing by replacing theliquid surrounding the rod with cold solvent, or in the case of anaqueous system, water at a temperature approaching freezing point orice-cold nitric acid. In the case of an aqueous system we have alsofound it possible to control the end point by measuring the change inconductivity of the water being used to unstuff.

(5) Drying, i.e., Removal of Solvent

Two problems occur in drying which affect the economics of the processand the quality of the product. These are cracking of the porous glassstructure and changes in the dopant composition profile. Cracking is astatistical problem and it is possible to have samples survive theprocess regardless of the drying procedure. However, in order to operateon a commercial scale, it is necessary to adopt a procedure whichminimizes cracking and thus improves the economics of the process. Sucha procedure should also preferably avoid profiles being altered in sucha way that dopant is transferred from the interior of the article towardthe surface as this is not usually a desirable profile. This results ina depression of dopant concentration in the center and an increase nearthe edges. As indicated above, the profile obtained is dependent on theunstuffing. Having achieved a suitable profile with solvent stillpresent in the porous structure, it is essential to dry, i.e., removesolvent in a way which will not alter that profile to an undesirablestate.

In an analysis of the drying process we have found that a number ofevents affect the process. These are:

(a) Gas evolution. The sources of gases can be the solvent, dopantdecomposition products and dissolved gases. If the gas evolution is toofast because of rapid heating or insufficient gas removal the resultingdifferential pressures in the pores can break the glass and/or carrydopant from inside of the article.

(b) Size change. As the bulk of the solvent is removed from the porousglass, a solvent layer may remain chemically bonded to the surface ofthe porous glass. We have observed this effect with water and found thelayer to persist up to high temperatures. This may also occur with othersolvents. As this layer is removed, the sample shrinks. With sufficientshrinkage difference across the porous structure, stresses can bedeveloped to the point of cracking.

(c) Dopant compound decomposition. The dopant as available in solutionis generally a compound which thermally decomposes. We have chosendopant compounds which decompose before the collapsing temperature. Thisdecomposition step is generally accompanied by a large evolution ofgases. It is generally desirable to control the heating rate while goingthrough the temperature range where decomposition occurs in order toprevent cracking and transport of dopant.

(d) Mass transport can occur at several stages in the drying process.When the article is dried initially, dopant which remains in solutioncan be transported to the surface and deposited there as the solvent isevaporated. If the solvent evaporates violently or boils evenprecipitated dopant can be displaced. After decomposition, if the dopantcrystals are small, they may be carried through the gas phase. If thedopant has a significant vapor pressure, dopant redistribution throughthe vapor phase may occur. If dopant becomes a liquid it mayredistribute according to gravity.

Outlined below is a preferred process for the suitable solution of theseproblems. The initial bulk removal of the solvent has to be performed bythe use of conditions where boiling does not occur; in the case ofaqueous solutions, we have used two procedures. One consists ofinitially drying the porous glass articles with precipitated dopant in adessicator (at atmospheric pressure) for 24 hours at 22° C. and thenplacing them in a drying oven. The second consists of placing thearticle under vacuum at temperatures below 10° C. and above the freezingpoint of the solution. We have found 4° C. for 24 hours to be convenientwhen using CsNO₃ in aqueous solution as a dopant. In order to minimizethe chances of cracking even further, we find that when using aqueoussolvents, it is convenient to subject the article to a final wash with anon-aqueous solvent which is non-reative with the glass. An example of asuitable solvent is methyl alcohol.

We have found it preferable to warm the articles which have beenmaintained below 10° C. under vacuum slowly to room temperature and tomaintain under vacuum at room temperature conveniently for about 24hours before the articles are transferred to a drying oven.

In the case of non-aqueous solutions of dopants, we have found itsuitable to place the articles under vacuum at room temperature for 24hours and then transfer to a drying oven. This significantly speeds upthe process as compared to an aqueous process.

In the drying oven, we have found it desirable to heat the samples to anupper drying temperature under vacuum at a rate below 30° C./hour,preferably 15° C./hour, since a slow heating rate significantly lowersthe cracking probability and avoids dopant redistribution.

The selection of a suitable slow heating rate will be dependent on theeconomics of the process. It may in some circumstances be cost effectiveto accept a higher breakage rate in order to increase the rate ofthroughput of articles through a processing system. However, anyincrease in heating rate must also be balanced against the increasedrisk of destroying the desired strengthening profile in the articleswhich are not cracked.

The upper drying temperature depends upon the porous glass matrix. It isdetermined by the fact that it should not be too high for the pores tocollapse and it should not be too low, otherwise the drying rate will betoo low to be economically useful. A suitable value can be found byfirst collapsing an undoped article and measuring its glass transitiontemperature, T_(g). The upper drying temperature is then preferablychosen to be in the range between 50° and 150° C. below the glasstransition temperature. We prefer to use a narrower range of 75° to 125°C.

The next stage of drying consists of holding the glass at or about thisupper drying temperature for periods of 5 to 200 hrs., preferably 40-125hrs. In this period, the glass may be held under vacuum or under aselected gas atmosphere depending on the optical properties (such ascolor absorption spectra) desired in the final product. At the upperdrying temperature one may use either reducing or oxidizing atmospheresin order (i) to control the valence states of multiple valence ions,particularly the transition metal ions, and (ii) to control the valencestates (either metallic or ionic) of the noble metals. For example, Cu²⁺gives blue color, Cu⁺ is red and Cu gives brown color. We have found itdesirable to pass gas around the article since this helps the dryingprocess. If the reduction of hydroxyl concentration is unimportant, thisholding period can be eliminated. The reduction of proton concentration(or moisture content) leads to (i) increased transmission in theinfrared over longer wavelengths, (ii) increased ultraviolettransmission and (iii) low d.c. conductivity and low a.c. loss. In ourpreferred procedure, we heat treat a porous glass article having a T_(g)of 725° C. at 625° C. (100° C. below the glass transition) for 96 hrs.while passing dry oxygen gas around the sample.

(6) Consolidation

Once the above drying process is complete, the article is now ready tobe collapsed. The article is raised rapidly in temperature to the pointwhere consolidation occurs. Once the pores are collapsed, consolidationis complete and the article is quenched to room temperature. Theconsolidation step must be conducted at atmospheric pressure or below ifthe article is to be further worked by reheating above the consolidationtemperature otherwise some gas evolution is likely to occur in reheatingand bubbles are formed.

We have found it desirable where the matrix is produced from aphase-separable glass to heat the porous glass samples under a reducedpressure (approximately 1/5 bar) up to 825° C. where consolidationoccurs.

The following examples illustrate the molecular stuffing aspect of theinvention but do not limit the invention. Examples I to V illustrate theuse of various concentrations of dopant in aqueous solutions in treatinga porous matrix which result in a glass on consolidation with differingoverall concentrations of dopant. The general procedure used forproducing the porous matrix from a phase-separable glass and thesubsequent treatment are shown in the paragraphs below and the actualnumbered examples illustrate the use of different dopants at a range ofconcentration, collapsing temperatures and final overall glasscomposition.

Melting and Forming

A glass having the composition in mol%; 4 NaO₂, 4 K₂ O, 36 B₂ O₃, 56SiO₂ was melted and stirred to produce a homogeneous melt from whichrods were drawn having a diameter in the range 0.7 to 0.8 cm., using acooling coil.

Heat Treatment

The drawn rods were heat treated at 550° C. for two hours to causephase-separation.

Etching before Leaching

Each rod was etched for 10 seconds in 5% HF followed by a 30 second washin water.

Leaching

The rods were leached at 95° C. with 3 N HCl containing 20% NH₄ Cl byweight, the time being chosen on the basis of previous trials so as toreach a stage where the rate of weight loss has dropped to almost nil.The leaching time of the rods used in these examples was chosen to be inexcess of 30 hours. During leaching, by providing a cold spot at 40° C.,the boric acid concentration in the leaching agent was kept below 50g/liter, thus speeding up leaching and avoiding possible re-depositionof boron compounds in the pores of the matrix. 40° C. is chosen so thatthere is no precipitation of NH₄ Cl from the leaching solution as thistemperature is above the saturation temperature of the NH₄ Cl present tomaintain a suitable amount in solution to reduce breakage drastically.

Washing

The leached material is washed with de-ionized water. The washing cycleis conveniently controlled by determining the concentration of iron inthe effluent. Washing is conveniently carried out at room temperatureusing 10 volumes of water to 1 volume of glass. We prefer in anon-continuous process to change the water about 6 to 8 times, giving awashing time of about 3 days.

Stuffing

With aqueous solutions of dopants (see Examples I to V below) we preferto move smoothly from the last washing stage to stuffing by simplyreplacing the water by the stuffing solution. This is done by drainingthe water from the last wash and filling the tube containing the porousrod with stuffing solution.

In Examples I through IV, the samples were removed from the stuffingsolution and cooled to 22° C. where the dopant precipitated partiallywith an amount equivalent to its aqueous solubility at 22° C. remainingin solution in the liquid filling the pores. (For example, 10 g Ba(NO₃)₂/100 ml solution remained dissolved in water in the pores after thermalprecipitation to 22° C. Similarly, 6 g H₃ BO₃ /100 ml solution remaineddissolved in water in the pores.) The remainder of the dopant wasprecipitated during the drying procedure which was commenced by placingthe porous article in an atmospheric pressure dessicator for 24 hours at22° C. Drying was then continued under vacuum in a furnace whosetemperature was raised at 15° C./hour to the upper drying temperature(as described above). This is determined in the manner described aboveand for the samples used in Examples I to VIII was 625° C.

Hold Time

The rods were then held at a temperature of 625° C. for 96 hrs. whilepassing dry oxygen gas around the rods.

Consolidation

On completion of the hold time, the temperature of the rods was raisedrapidly to a high temperature (given in each example) where collapsingtook place under a reduced pressure of oxygen (approximately 1/5 bar).

In Example I, BaO is used as dopant. It is immiscible with the baseglass at the collapsing temperature and causes the glass to becomeopalescent provided that it is not reworked at higher temperatures. InExample II, B₂ O₃ is used as dopant. B₂ O₃ reduces the glass transitiontemperature and hence is desirable for strengthening using the T_(g)method. In example 3, PbO is used as dopant. PbO decreases the glasstransition temperature and increases the expansion coefficient.

EXAMPLE I Molecular Stuffing with BaO Dopant Ba(NO₃)₂ in Water

    ______________________________________                                                                    Con-                                                   Dopant   Stuff-        solida-                                                                              Compos-                                         gms/     ing     Details                                                                             tion   ition                                           100cc    Time:   Temp. Temp.  Mole                                       Rod  of H.sub.2 O                                                                           Hours   °C.                                                                          °C.                                                                           %       Wt %                               ______________________________________                                        1    12       4       85    820    6.0 B.sub.2 O.sub.3                                                                   6.86                                                                  93.11 SiO.sub.2                                                                       91.10                                                                 .82 BaO                                    2    18       4       85    820    6.0 B.sub.2 O.sub.3                                                                   6.79                                                                  92.73 SiO.sub.2                                                                       90.18                                                                 1.22 BaO                                                                              3.03                               3    24       4       85    830    6.00 B.sub.2 O.sub.3                                                                  6.72                                                                  92.35 SiO.sub.2                                                                       89.28                                                                 1.62 BaO                                                                              4.01                               ______________________________________                                    

EXAMPLE II Molecular Stuffing with B₂ O₃ Dopant H₃ BO₃ in Water

    ______________________________________                                                                    Con-                                                   Dopant   Stuff-        solida-                                                                              Compos-                                         gms/     ing     Details                                                                             tion   ition                                           100cc    Time:   Temp. Temp.  Mole                                       Rod  of H.sub.2 O                                                                           Hours   °C.                                                                          °C.                                                                           %       Wt. %                              ______________________________________                                        Com-                               6.09 B.sub.2 O.sub.3                                                                  7                                  par-  0       --      --    820    93.88 SiO.sub.2                                                                       93                                 ison                                                                          Rod                                                                           4    12       4       85    815    7.73 B.sub.2 O.sub.3                                                                  8.83                                                                  92.27 SiO.sub.2                                                                       91.17                              5    18       4       85    815    8.51 B.sub.2 O.sub.3                                                                  9.71                                                                  91.49 SiO.sub.2                                                                       90.29                              6    24       4       85    810    9.2 B.sub.2 O.sub.3                                                                   10.58                                                                 90.72 SiO.sub.2                                                                       89.42                              ______________________________________                                    

EXAMPLE III Molecular Stuffing with PbO+B₂ O₃ Dopant Pb(NO₃)₂ and H₃ BO₃in Water

Rod #7

Doped with 40 gms Pb(NO₃)₂ and 10 gms H₃ BO₃ per 100 cc of H₂ O at 85°C. for 12 hours. Collapsed at 825° C.

EXAMPLE IV Molecular Stuffing with BaO+B₂ O₃ Dopant Ba(NO₃)₂ and H₃ BO₃in Water

Rod #8

Doped with 12 gms Ba(NO₃)₂ and 6 gms H₃ BO₃ per 100 cc of H₂ at 85° C.for 4 hrs. Collapsed at 830° C.

EXAMPLE V

The above examples all relate to uniform doping of a rod. As describedabove, it is possible once a rod has been left for sufficient time todiffuse the dopant solution into the pores, to then reduce theconcentration in the outermost part of the rod so as to achieve acomposition profile in the collapsed rod. Two porous rods were producedby the procedure outlined above, and were immersed for more than fourhours at 95° C. in an aqueous solution of CsNO₃ with a concentration of120 g CsNO₃ /100 ml solution. The rods were then transferred to water at95° C., and left in the water for periods of 11, and 20 minutesrespectively. Each rod at the expiration of the time in water wasimmersed in water at 0° C. for 10 minutes to cause thermal precipitationof CsNO₃. The rods were then treated to remove solvent and collapsed inthe manner described above. The composition profiles as measured byrefractive index are shown in FIG. 7.

EXAMPLE VI

Several porous rods produced by the method described above were immersedin a series of solutions of CsNO₃ and Cs₂ CO₃ as described in Table Vbelow for more than four hours at 95° C. These were then unstuffed toproduce a step profile. The time required for unstuffing was determinedusing FIG. 6 (i.e., for rod #13 the time for which y(t_(o))=0.50 is 300min.). The stuffed rods were unstuffed by immersion in ice water for 300min. and the water was removed and rods were collapsed as describedabove. The resulting Cs₂ O concentration measured in terms of refractiveindex, in the center of the rod, is listed in Table V.

                  TABLE V                                                         ______________________________________                                                                        at center                                     Rod Number                                                                             Stuffing Solution      of rod                                        ______________________________________                                         9       20 grs Cs.sub.2 CO.sub.3 /100ml H.sub.2 O                                                            1.462                                         10       30 grs Cs.sub.2 CO.sub.3 /100ml H.sub.2 O                                                            1.475                                         11       75 grs Cs.sub.2 CO.sub.3 /100ml H.sub.2 O                                                            1.488                                         12       60 grs CsNO.sub.3 /100ml aqueous solution                                                            1.475                                         13       120 grs CsNO.sub.3 /100ml aqueous solution                                                           1.486                                         ______________________________________                                    

EXAMPLE VII

As discussed previously, we emphasized the importance of drying at aslow rate between temperatures near 0° C. and 600° C. when using rodsstuffed with dopants. Here we show an example of differences incomposition profiles due to different heating rates. Several stuffedporous rods as used in Example V were unstuffed to produce steppedprofiles as in Example VI. After thermal precipitation, the rods weredried under vacuum at 4° C. for 24 hours. They were then heated undervacuum at rates of 50,30 and 15° C./hr respectively. The resultingcomposition profiles measured in terms of refractive index profiles areshown in FIG. B. Heating at rates above 100° C./hr caused appreciablebreakage in the rods.

EXAMPLE VIII

After leaching and washing as described above in the introduction toExamples I-IV, a porous rod is immersed for 4 hours in a solutioncontaining 120 g of Cs(NO₃) per 100 cc of water. It is then dried in adessicator for 24 hours at room temperature. The sample is uniformlystuffed and in order to introduce a profile, it is then washed at 4° C.in water for two hours and then in 3 N HNO₃ for 30 minutes. This isfollowed by drying in a vacuum at the same temperature. Once the bulk ofthe water is removed, it is slowly dried by progressively raising thetemperature as described in our preferred procedure. At intermediatetemperatures, the CsNO₃ decomposes into Cs₂ O and various nitrogenoxides. When the sample changes from white to clear, the consolidationis complete and the sample is removed from the furnace. When analyzed atroom temperature, the sample has a surface stress in excess of 10,000psi and an expansion coefficient of 3.1×10⁻ 6 /°C. and a use temperaturein excess of 700° C. The thickness of the compressive layer is over 20mils. This final rod is stronger than a similar one made of fusedsilica.

EXAMPLE IX

As indicated above, it is possible to vary widely the choice of dopant,solvent and conditions of operation during stuffing and unstuffing andthe combinations and permutations of these parameters in order toachieve a desired end result, or modification of processing conditions.We have given guidance to the man practiced in the art; this exampleillustrates some of the permutations and combinations we have foundsatisfactory. The porous rods used were all produced by the generalprocedure described above and solvent removal and heating carried outunder our preferred conditions.

The results obtained are given below in Table VI. The columns in thistable give the following information:

Column 1: Dopant used.

Column 2: Concentration of dopant/100 ml solution.

Column 3: Solvent, i.e., solvent used for initial stuffing.

Column 4: Temperature in °C., and time taken for initial stuffing.

Column 5: Solvent A--this is the solvent used to reduce theconcentration and produce composition variation, and also to causeprecipitation of the dopant.

Column 6: Temperature in °C. and time taken for precipitation andvariation in composition profile.

Column 7: Solvent B is used where appropriate to adjust dopantdistribution in matrix. By causing further precipitation before solventremoval begins so as to enable a more thorough decrease in dopantconcentration near glass surface.

Column 8: Temperature in °C. and time taken for adjusting dopantdistribution by further solvent treatment.

Column 9: Indicates temperature at which drying commenced in °C., and by"V" or "A" whether drying in vacuum (V) or in dessicator at atmosphericpressure (A) for the first stage.

Column 10: Gives the composition in terms of the index of refractionwhere measured.

In the table,

Line 1: is the same as Rod #13 in Example VI above and is included forcomparison with line 2, where by including a further treatment withmethanol and water, while using the same stuffing solution, thedifference in index is increased.

Line 3: demonstrates how by replacing one compound by another, in thiscase CsNO₃ by Cs₂ CO₃ because of higher solubility, stuffing can becarried out at room temperature.

Lines 4-9: illustrate the use of different dopant and solventcombinations.

Lines 10 & 11: show the use of a mixture of dopants.

Line 12: illustrates the use of reodymium nitrate as a dopant, and ofthe use of an organic solvent. Doping with neodymium (or othertransition elements) can be used to produce colors in the strengthenedarticle.

                                      TABLE VI                                    __________________________________________________________________________    1       2     3     4    5     6     7     8    9     10                      Stuffing                                                                               Weight          Precipitation          Drying                                per 100 ml  Temp. &    Temp. &     Temp. &                                                                            commence                                                                            Index                   Dopant  solution                                                                            Solvent                                                                             time Solvent A                                                                           time  Solvent B                                                                           time at (°C.)                                                                     of                      __________________________________________________________________________                                                          Refraction              1. Cs(NO.sub.3)                                                                       120 g water 95° C.-                                                                     water 0°C.-3 hr                                                                    --    --   4° C.(V)                                                                     center 1.486                                16 hr                             edge 1.464              2. Cs(NO.sub.3)                                                                       120 g water 95° C.-                                                                     water 0° C.-3 hr                                                                   30% water                                                                           0° C.-3                                                                     4° C.(V)                                                                     center 1.489                                16 hr            70%              edge 1.458                                                   methanol                                 3. Cs.sub.2 (CO.sub.3)                                                                50 g  water 22° C.-                                                                     92% meth.                                                                           22° C.-3 hr                                                                  52%   22° C.-3                                                                    22° C.(V)                                                                    1.487                                       16 hr                                                                              8% 1-pro-   methanol                                                                            hr                                                          panol       48% 1-pro-                                                                    panol                                    4. Na(NO.sub.3)                                                                       17.5 g                                                                              water 25° C.-                                                                     --    --    --    --   25° C.(A)                                                                    1.465                                       4 hr                                                      5. H.sub.3 BO.sub.3                                                                   18 g  water 85° C.-                                                                     water 22° C.                                                                       --    --   22° C.(A)                                                                    1.457                                       4 hr       10 min                                         6. Pb(NO.sub.3).sub.2                                                                 40 g  water 22° C.-                                                                     30% H.sub.2 O                                                                             5% water                                                                            22° C.-3                                                                    22° C.(V)                                                                    1.465                                       16 hr                                                                              70% meth-   95% metha                                                                           hr                                                          anol        nol                                      7. La(NO.sub.3).sub.3                                                                 50 g  ethanol                                                                             22° C.-                                                                     15% etha-                                                                           22° C.-3 hr                                                                  10% etha-                                                                           22° C.-3                                                                    22° C.(V)                                  16 hr                                                                              nol*        nol*  hr                                                          85% diethyl 90% diethyl                                                       ether       ether                                    8. Ba(NO.sub.3).sub.2                                                                 24 g  water 85° C.-                                                                     water 22° C.-3 hr                                                                  --    --   22° C.(A)                                  4 hr                                                      9. Al(NO.sub.3).sub.3                                                                 60 g  water 25° C.-                                                                     --    --    --    --   22° C.(A)                                                                    +                          9 H.sub.2 O      4 hr                                                      10.                                                                              Ba(NO.sub.3).sub.2                                                                 12 g  water 85° C.-                                                                     water 22° C.-                                                                      --    --   22° C.(A)                                                                    +                          +                4 hr       10 min.                                           H.sub.3 BO.sub.3                                                                   6 g                                                                      Pb(NO.sub.3).sub.2                                                                 40 g  water 85° C.-                                                                     water 22° C.-                                                                      --    --   22° C.(A)                                                                    +                          H.sub.3 BO.sub.3                                                                   10 g        12 hr      10 min.                                           Nd(NO.sub.3).sub.3                                                                 20 g  90% di-                                                                             22° C.-                                                                     di-   22° C.-3 hr                                                                  --    --   22° C.(A)                                                                    1.464                                 ethyl 16 hr                                                                              ethyl                                                              ether      ether                                                              10%                                                                           ethanol                                                         __________________________________________________________________________     *In this case the stuffed rod was immersed in solution B for 11/2 hr, the     in solutions A & B for 3 hrs. respectively.                                   +These glasses scattered light when collapsed and turned clear when pulle     into fibers.                                                             

Articles in the shape of a rod can be strengthened by this invention.Such strengthened rods can be used as preforms for pulling strong fibersto be used as reinforcement materials.

If a flat sheet of glass is strengthened by this invention, this sheetcan be used as a window which can withstand large gradients intemperature (as caused by sun loading) and simultaneously reasonablepressure differentials across it as caused, for example, by high winds.

Molded sheets of this material can be used as stove tops since it wouldwithstand the thermal shock of the heating element as well as themechanical shock of abuse and wear.

An advantage of this invention over the thermal temperature method ofstrengthening glass is the ease with which it can be cut afterstrengthening. This is so because the center tension can be kept muchlower than in thermal temperature for equivalent surface compression.The reason for this is the improved control of the stress profilepossible with this present invention. The surface layer may have athickness of at least about 10 mils and preferably 20 mils in order toprevent loss of strength from scratches. For example the rod in ExampleVIII can be cut with a diamond saw without shattering and yet withstandscratches. Because in this invention, whereas the interior may containonly 80% of silica (preferably 85%), the surface layer contains inexcess of 85% of silica (preferably in excess of 90%), articles made inthis fashion are relatively inert chemically and can be used for examplein bottles for baby food and for blood plasma. In both these examplesthe leaching out of sodium is a problem when ordinary soda lime glass isused.

Although presently preferred embodiments of the invention have beenshown and described with particularity, it would be appreciated thatvarious changes and modifications may suggest themselves to those ofordinary skill in the art upon being apprised of the present invention.It is intended to encompass all such changes and modifications as fallwithin the scope and spirit of the appended claims.

What is claimed is:
 1. Process for producing a rigid glass product whichcomprises washing a porous silicate glass with sodium hydroxide, thenimmersing the washed porous silicate glass in a liquid solution of adopant in a liquid solvent therefor to stuff the pores of said washedglass with said solution, subsequently removing said solvent from saidpores and thereafter heating to collapse said pores.
 2. Process forproducing a rigid glass product which comprises washing a poroussilicate glass with sodium hydroxide, then immersing the washed poroussilicate glass in a liquid solution of dopant in a liquid solventtherefor to stuff the pores of said washed glass with said solution, andthereafter heating to remove said solvent from said pores and then tocollapse said pores.